The present invention concerns a method and a plant for the preparation of feedstuff pellets by spraying relevant additives in liquid form on a continuous thin layer of the feedstuff pellets. It is generally desirable that in feedstuff pellets there should exist a certain addition of qualified materials such as enzymes, flavouring and vitamins, and it has previously been known to add such materials in solid particle form to the basic material, which is prepared for feeding purposes by pellet pressing. Various recipes can be used, all depending on how the pellets are to be used, and by such mixing in the basic material a precise dosing can be achieved, e.g. based on weighing.
However, most of the relevant additives have proved to be vulnerable to the actual pellet formation process, in that considerable heat is developed when these are pressed, whereby the materials are rendered more or less ineffective or, where some are concerned, are completely ruined. Because of this, an alternative principle of dosing has been employed, i.e. treating the finished and thereafter cooled pellets with the relevant materials in a dissolved or suspended liquid form. Various techniques have been put forward for this purpose, though with inherent disadvantages with regard to capacity, uniformity or dosing precision, and also an undesired crumbling and formation of dust.
It is sufficient here to mention just one of the suggested principles, i.e. the providing of a cascade of the pellet material distributed in a thin layer out over an inclined chute, whereby the quickly down-sliding layer can pass a spraying station from which the layer of pellets can be dosed with the additives through a suitable nozzle arrangement. It is precisiely when this layer is thin and when it is subsequently conveyed for collection, e.g. on a conveyor leading sideways away from the chute, that a certain mixing of the material will occur, whereby it is possible for the product sprayed on the thin layer to spread in a predominantly even manner in the finally collected pellet mass.
There are, however, certain distinct problems connected with this known technique, i.e. partly with regard to the dosing precision of the sprayed-on additives, in that this dosing should, of course, be carefully adapted to suit the current amount of material, i.e. layer thickness and flow-rate of the thin layer of pellets, and partlyxe2x80x94of a more practical naturexe2x80x94with regard to the construction height of the plant necessary for establishing a cascade of the thin layer with such a high speed that it is at all possible to operate with a capacity which is of interest in commercial practice.
Both of these problems are quite significant in practice. Consequently, it can clearly be ascertained that the construction height necessary for producing a flow-rate of usable degree will exceed a normal storey height, i.e. the installation of the plant will demand special site conditions. The creation of the thin-layer formation of the pellets could be effected with lower height requirements by feeding the material to a quickly-moving, horizontal conveyor, but for various reasons this solution is not attractive.
Also the second of the above-mentioned problems, i.e. the dosing precision of the additives, is of decisive significance. A plant which should be able to operate with pellet materials of several different kinds will not necessarily deliver these materials in a completely even cascade at uniform speeds and layer thicknesses. Therefore, it can be extremely difficult to determine a correct dosing for the spraying of the pellets. It should be borne in mind that in practice it is current to operate with quite a high capacity, typically 10-100 tons per hour, and with a relatively small addition of liquid, e.g. merely 50 ml-10 litres per ton of material. It is not desirable to operate with additives in specially thin solutions, since the pellets should not be moistened more than is strictly necessary, and for precisely this reason the additives must be held at a minimum, also because in some cases the additives are extremely expensive.
To this can be added that quite strict demands are made on the uniformity of the application both in general and to a particular degree in connection with feedstuff mixtures intended for smaller creatures such as poultry. No useful purpose is served in effecting a dosing of, for example, 200 ml per ton, if the distribution is not good enough to ensure that, at each feeding with a relatively modest amount of feedstuff, the animals will at least receive a more or less representative part of the additives.
It is for this reason so important out of regard for capacity that the dosing of the additives is effected on a quickly-transported flat flow of the material, so that the sparsely dosed additives can be applied to as many of the material particles as at all possible.
It is easy to prescribe that a relevant dosing will be, e.g. 400 ml per ton, but in practice great difficulties have arisen in controlling the current dosing in order to achieve such a desired result while at the same time achieving uniform distribution, and from both the consumer and the producersr""s side it has been accepted that appreciable variations can arise with regard to both under- and overdosing to the detriment of the one and the other party respectively, and without any ideal consequences for the animals to be fed.
With the present invention, the primary intention is to improve the dosing precision of the sprayed-on additives, and in such a way that a distribution of xe2x80x9cthe dosing per tonxe2x80x9d can be effected with appreciably increased precision and good distribution to the advantage of both consumer and manufacturer. While deviations of 10-20% have traditionally had to be accepted, with the invention it will be possible to operate with deviations of only approx. 2%.
One of the uncertainties involved in the use of the down-flow of a thin layer of material on a chute will be that, due to the frictional influences which occur, the different kinds of materials can develop different speeds in their passage through the spraying station, which also applies for one and the same material type, depending on its specific gravity and degree of comminution. Even though these differences in speed are not overwhelmingly great, they can still give rise to appreciable inaccuracies in the effective liquid dosing. A controlled graduation of the spraying capacity could be arranged depending on a measurement of the down-flow speed in or close to the spraying station, but for various reasons this is difficult to realize in practice.
With the present invention it has been found desirable to retain the principle concerning the spraying of a quickly-moving thin layer of material, the reason being that this provides the possibility of good distribution of the expensive elements in the material, but it has also been found desirable to depart from the establishing of a conventional down-flow of the material, in that use is made instead of an ejection arrangement such as a bucket wheel which can eject the flat flow at a controlled desirable speed, so that the flow of material can be fed to the spraying station without having to pass along any longer, friction-generating acceleration section.
There are hereby achieved two substantial advantages, i.e. partly that the speed of the material in the spraying station will be well-controlled regardless of the kind and nature of the material, and partlyxe2x80x94purely practicalxe2x80x94that the relevant plant can be built with a substantially reduced construction height conventional plants based on gravitational acceleration of the material must necessarily be of a construction height which exceeds normal storey height, otherwise the fall-speed generated will be too low, and for the practical application of the invention it is extremely advantageous that a plant according to the invention can be dimensioned in such a manner that even with the quite great capacity it can still be installed in premises with normal headroom.
With the invention, there is thus provided an immediate solution to the first-mentioned problem regarding the flowspeed of the material in or through the spraying station, but the invention goes further than this, i.e. in recognizing that the desired high degree of dosing accuracy will not be achieved merely by this arrangement. As discussed, there can, for example, arise variations in the specific gravity of the material, which will be without influence on the ejection velocity or layer thickness of the material flow, but which, however, must be taken into account in achieving an accurate xe2x80x9cdozing per tonxe2x80x9d. It will naturally be a possibility to carry out a continuous determination of the specific gravity of samples of the handled material, and an adustment of the spraying equipment depending on the sampling results. However, this must be seen to be a purely theoretical measure due to the associated costs.
However, with the invention it has been found possible to take the weight factor into consideration, and even in two quite different ways:
1 Continous flow weighing
By using a short, inclined chute between the ejection arrangement and the spraying station, it is possible to introduce a path-section at which a continuous determination of the weight of the flat flow of passing material can be carried out. When this determination is compared in a computer unit with information concerning the desired liquid dosing for the relevant material, it will be possible on this basis to effect a control which results in the liquid dosing per unit of weight of the material being regulated to the correct amount. This can be done in two ways, i.e. either by a regulation of the spraying capacity or a regulation of the ejection speed of the material flow, in that here the possibility of regulating the thickness of the material is not considered because it is not desirable to enter into any compromise with regard to the uniform distribution of the additives.
In this connection it is important that the ejection arrangement is configured as a volume-dosing unit, so that purely from the control point of view it is known what amount of material is currently separated in a thin-layer flow, since the speed of this is not, after all, the only decisive factor. If the weighing arrangement registers an increased unit weight of the material flow, this will thus be an indication of a change in the specific gravity and not in the speed, whereby it is possible to generate a control signal which in an unambiguous manner can bring about either a required up-regulation of the liquid dosing or a required down-regulation of the material flow, with associated slight reduction in the speed thereof.
2. Differential weighing-of the material supply:
By a continuous weighing of the amount of material which is at disposal of the ejection arrangement, the speed with which the material is dispensed from the supply can be ascertained, e.g. the supply held in a funnel-shaped container above the ejection arrangement. The mass flow will be given as the weight difference as a function of time. Strictly speaking, it is hereby of no consequence whether the dispensation of the material takes place in a volume-dosed manner, merely providing that the delivery takes place with good uniformity and at the known speed. Moreover, a flow-weighing will not be necessary. When a change occurs in the specific gravity of the material, this will hereby automatically be taken into consideration, in that by the volumetric even dispensation there will be released a correspondingly-changed weight amount per unit of time, which can then be signalled to the regulation unit either for the spraying station or the ejection arrangement.
With continuous operation, this weight-determined control must be suspended during periods in which the container is refilled, but this can take place with such great capacity that such periods will be quite brief. After the refilling, the increased height of the material in the container can serve in such a way that the ejection arrangement will operate with a greater degree of filling, i.e. it will eject an increased flow, but this will be registered already at the first operative differential measurement of the container weight. Moreover, the control unit can be programmed, on the basis of empirical data, to exercise an approximately correct control in the meantime. It will be a further possibility to arrange a more even delivery of the material via a belt weigher, but this gives immediate rise to the problem that there is hereby a distinct risk of a reduction in the high accuracy desired for the effective liquid dosing.
As mentioned, a regulation of the spraying can be effected on the basis of the weighing signals. This is possible within quite a wide area of regulation but, with relatively low liquid dosages, the use of normal spraying nozzles can give rise to problems regarding full coverage of the breadth of the thin layer of material. If it is desired with one and the same plant with more or less high maximum capacity for coarse feedstuffs, e.g. 50 tons per hour, also to be able to a handle fine feedstuffs, e.g. for fish, where an extremely good distribution of the liquid is demanded, it is relevant to ensure that the liquid dosing is not down-regulated so much that the above-mentioned full coverage disappears. A is choice can thus be made to operate constantly with the least possible effective dosing, but on the other hand to carry out the whole regulation on the capacity of the ejection arrangement.
When using a simple ejection arrangement such as a bucket wheel, a regulation of the mass flow will to a wide extent be related with a change in the ejection speed. Conventional fall-flow plants operate with a relatively narrow range of speed, whereby the proportionality is approximated between, e.g,. a flow increase and an increase in the liquid dosing. with the invention, operations can well be effected with a large area of variation for both the mass flow and the speed, e.g. corresponding to 25-100% of the capacity of the ejection unit, but with a change in the flow regard must be paid to the fact that when controlling the liquid dosing, the speed can also be changed to a noticeable degree, generally for damping of the regulation requirement for the liquid dosing.
It must be mentioned that the flow-weight determination as an alternative to the two aforementioned methods can also be effected by continuous weighing of the ejection wheel itself, even though this would immediately appear to be an unrealistic possibility.
It must also be mentioned as an advantage of the invention that the material is handled in such a manner that dust formation is avoided to the widest possible extent, e.g. in that changes in the mass flow are accepted without the use of mechanical throttling of the flow, and moreover by an extremely careful feeding of the material. Feedstuff pellets are potentially dust-forming, and it is characteristic for the dust that it not only absorbs liquid better than the pellets themselves, but is also rejected by the animals as feedstuff. Moreover, it can actually be currect practice to sift out the dust before the pellet material is delivered, and that part of the dosed liquid which has been absorbed by the dust can thus be totally useless.
A plant according to the invention can be configured in a totally enclosed manner to exclude any risk of the material being contaminated.